RV Digestit. Google it or check your local RV place. Follow the directions on the bottle. Cleans EVERYTHING. Black tank, gray tank etc. I've only used it as a tank chemical, but we used it on a friend of mines where his gauges weren't working at all. There was crap that came out of that tank from probably 2 years before. It eats and breaks loose everything. All his gauges worked after that.
John and Rebecca Dickson
Emma-8 / Little John-5 / Iva-1+
2013 Ford F-350 Lariat FX4, CC LB PSD, DRW
2015 Jayco Jay Flight 28BHBE (#8)